It is known in the art relating to unit fuel injectors for diesel engines to provide a positive displacement plunger pump with a controlled output to pump fuel at high pressure through a spray tip directly into an associated combustion chamber for combustion therein. A well known feature of such injectors is the provision of a flat check valve to prevent the back flow of fuel or combustion gases from the combustion chamber and spray tip into the plunger pump location. A known type of flat check valve is in the form of a small disk having the outer edges scalloped to provide flow passages for fuel when the valve is open and seated upon an annular seat open internally to a flow chamber. Such check valves have been used for many years in some of the unit fuel injectors made by General Motors and subsequently by Diesel Technology Corporation, including those supplied for use in the well known Electro-Motive Division (EMD) diesel and dual fuel engines manufactured for railroad locomotives and other applications.